Magnetic thin film disks with a nonuniform composition

ABSTRACT

A method of influencing variations in composition of thin films is described. The elemental plasma field distribution in sputtering systems is manipulated by generating a nonuniform electric field along a surface of the substrate to alter the composition by differentially re-sputtering the target elements. The nonuniform electric field is applied by one or more electrodes in contact with a conductive surface or by using an RF bias signal. The nonuniform electric field is used to modulate the kinetic energy of the ions generated in the plasma which strike the thin film&#39;s surface. Since the kinetic energy and the mass of the sputtering gas ions and neutrals affect the re-sputtering rate, the nonuniform electric field differentially affects the elements being deposited according to mass. By applying varying electric potentials at a plurality of points on a conductive surface of a substrate, the electric field across the surface of the substrate can be modulated in a variety of patterns. For example, the field can be varied along the circumferential and/or radial direction of a disk. In the preferred embodiment a radial voltage gradient is applied to a conductive surface of a disk on which a magnetic thin film is being formed to radially modulate the platinum content of the magnetic film. Modulating the radial platinum content in turn modulates the radial coercivity.

FIELD OF THE INVENTION

[0001] The invention relates to magnetic thin film disks and methods fortheir fabrication and more particularly to magnetic thin film mediahaving nonuniform magnetic properties due to nonuniform composition andmethods for creating the nonuniform composition.

BACKGROUND OF THE INVENTION

[0002] A typical prior art head and disk system 10 is illustrated inFIG. 1. In operation the magnetic transducer 20 is supported by thesuspension 13 as it flies above the disk 16. The magnetic transducer 20,usually called a “head” or “slider,” is composed of elements thatperform the task of writing magnetic transitions (the write head 23) andreading the magnetic transitions (the read head 12). The electricalsignals to and from the read and write heads 12, 23 travel alongconductive paths (leads) 14 which are attached to or embedded in thesuspension 13. Typically there are two electrical contact pads (notshown) each for the read and write heads 12, 23. Wires or leads 14A,14B, 15A, 15B are connected to these pads and routed in the arm 13 tothe arm electronics (not shown). The magnetic transducer 20 ispositioned over points at varying radial distances from the center ofthe disk 16 to read and write circular tracks (not shown). The disk 16is attached to a spindle 18 that is driven by a spindle motor 24 torotate the disk 16. The disk 16 comprises a substrate 26 on which aplurality of thin films 21 are deposited. The thin films 21 includeferromagnetic materials in which the write head 23 records the magnetictransitions in which information is encoded.

[0003]FIG. 2 illustrates a section of a prior art disk 16. Theconventional substrate 26 is a conductive disk of AlMg with anelectroless coating of NiP which has been highly polished. The thinfilms 21 on the disk 16 conventionally include a chromium or chromiumalloy underlayer (s) 31 which is deposited on the substrate 26. Therecording layer (s) 33 is (are) based on various alloys of cobalt,nickel and iron. For example, a commonly used alloy is CoPtCr.Additional elements such as tantalum and boron are often used tomagnetically isolate the grains. A protective overcoat layer 35 is usedto improve wearability and corrosion. While instructive, the three filmdisk described above does not exhaust the possibilities. Various seedlayers (not shown), multiple underlayers (not shown) and laminatedmagnetic films (not shown) have all been described in the prior art. Inaddition, other materials besides AlMg are utilized as substrates.

[0004] When a magnetic disk 16 is designed for a future disk drive 10, atarget coercivity range is determined based on the overall systemrequirements. For example, an upper limit on the coercivity is set bythe write head's 23 ability to induce transitions in the magnetic film33. Therefore, part of the disk designer's task is to obtain a specificcoercivity range rather than the highest possible coercivity. One methodof adjusting the coercivity follows from the fact that the platinumcontent of the magnetic film 33 is known to directly affect the filmcoercivity. Within limits, a marginal change in the platinum contentwill directly affect the coercivity by a predictable amount. Thecomposition of the magnetic film 33 mirrors the composition of thesputtering target to a good level of accuracy, so a marginal increase inthe platinum content in sputtering target is reflected in the depositedfilm.

[0005] At a macro level it is desirable for the coercivity of themagnetic film 33 to be fairly uniform both radially andcircumferentially. However, there are factors at work in a disk drivewhich may make it desirable to have subtle radial gradients in thecoercivity. For example, at constant rotation speed, the flying heightof the transducer 20 above the disk 16 may vary from the inner diameter(ID) to the outer diameter (OD) of the disk 16. The flying heightdirectly affects the field strength generated by the write head 23 inthe magnetic film 33. The linear velocity (for constant rpm) is higherat the OD than the ID. This implies that the head flies higher at the ODas compared to the ID. This presents a writability problem at the OD.

[0006] Films are grown by sputtering from alloy targets whosecompositions are optimized to provide the desired magnetic properties.The target material is held at a negative voltage to provideacceleration for the positively charged sputter gas ions (typically Ar).The ground potential for this arrangement is normally the chamber walls.The substrate is not grounded. Current-art sputtering systems used forfabricating magnetic disks 16 provide also the capability of providingnegative or positive bias to the disk substrate. The voltage used istypically on the order of −300 volts.

SUMMARY OF THE INVENTION

[0007] A method of influencing variations in composition of thin filmsis described. The elemental plasma field distribution in sputteringsystems is manipulated by generating a nonuniform electric field along asurface of the substrate to alter the composition by differentiallyre-sputtering the target elements. The nonuniform electric field isapplied by one or more electrodes in contact with a conductive surfaceor by using an RF bias signal. The nonuniform electric field is used tomodulate the kinetic energy of the ions generated in the plasma whichstrike the thin film's surface. Since the kinetic energy and the mass ofthe sputtering gas ions and neutrals affect the re-sputtering rate, thenonuniform electric field differentially affects the elements beingdeposited according to mass. By applying varying electric potentials ata plurality of points on a conductive surface of a substrate, theelectric field across the surface of the substrate can be modulated in avariety of patterns. For example, the field can be varied along thecircumferential and/or radial direction of a disk. In the preferredembodiment a radial voltage gradient is applied to a conductive surfaceof a disk on which a magnetic thin film is being formed to radiallymodulate the platinum content of the magnetic film. Modulating theradial platinum content in turn modulates the radial coercivity.

BRIEF DESCRIPTION OF THE FIGURES

[0008]FIG. 1 is a symbolic illustration of the prior art showing therelationships between the head and associated components in a diskdrive.

[0009]FIG. 2 is an illustration of one type of prior art layer structurefor a magnetic thin film disk.

[0010]FIG. 3 is an illustration of a mechanical setup for applying biasto a disk substrate during sputtering.

[0011]FIG. 4 is an illustration of a midline section view of the carrierand disk shown in FIG. 3 along the line marked IV.

[0012]FIG. 5 is an illustration of an electric field gradient along themidline section of the disk and carrier in the sputtering chamberaccording to the invention.

[0013]FIG. 6 is a plot of experimental data showing the platinum contentof a disk according to the invention plotted by radial distance.

[0014]FIG. 7 is a plot of the experimental data showing the coercivity(Hc) versus the platinum content for the disk of FIG. 6.

[0015]FIG. 8 is an illustration of a nonuniform electric field along themidline section of a substrate applied with a plurality of electrodesaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

[0016]FIG. 3 is an illustration of one method of applying a voltage biasto a disk 16 while the thin films are being sputtered. The disk 16 ismechanically supported by a conductive carrier 41 which is connected toa voltage source 43. The voltage range of the voltage source 43 shouldbe approximately −150 to −600 v. Typically both sides of the disk 16 areused so the carrier 41 needs to be designed to support the disk 16without obscuring any more of either surface than is necessary. Thereare clearly numerous ways to achieve this and the details are outside ofthe scope of the invention. For the purpose of describing the preferredembodiment of the invention, it is only necessary to have the disk 16 incontact with an electric conductor in a substantially continuous wayaround the outer circumference or alternatively around the innerdiameter. The carrier 41 can be a large movable rack with supports formany disks 16 which can travel through a pass through sputtering systemor it can be a designed for stationary support of a single disk 16.

[0017]FIG. 4 is a midline section view of the disk 16 and carrier 41 ofFIG. 3 along line IV. This section shows that carrier 41 supports disk16 by a narrow lip 42 which extends around the entire circumference ofthe hole in carrier 41 which supports the disk 16. This arrangementleaves substantially all of both sides of disk 16 exposed forsputtering. More importantly, however, because the outer circumferenceof the disk 16 is continuously in contact with the conductive carrier 41the electric field which results from the negative bias voltage isuniform around the circumference of the disk 16.

[0018]FIG. 5 is the same midline section of FIG. 4, but in this drawingthe disk 16 and carrier 41 are illustrated in the sputtering chamber 50which contains plasma 43 which contains the sputtering gas ions and thetarget sputtered elements. In the case where a CoCrPt magnetic film isbeing deposited using argon as the working gas, the plasma would containpositive ions and neutrals of the sputtering gas as well as neutral andcharged Co, Pt and Cr species. The electric field generated around thedisk 16 is illustrated by the arrows which are arranged perpendicularthe disk's planar surface. Only one side of the disk 16 is decoratedwith the arrows in order to simplify the illustration and provid roomfor the element numbers and labels, but the field on the undecoratedside is symmetric with what is shown. The size of the arrow indicatesthe strength of the electric field relative to a selected baseline. Inabsolute terms the variation in the electric field is small, butnevertheless significant. The electric field is shown as being strongestat the circumference of the disk 16 and monotonically declining to thelowest value at the inner diameter.

[0019] One effect of the electric field is accelerate the positive ionsin the plasma 43 toward the disk 16. In the absence of the electricfield bias, positive ions are only accelerated towards the targetmaterial. Application of negative bias to the substrate results inpositive ion bombardment of the growing thin film surface. Thepredominant impinging species on the growing thin film surface arepositively charged Ar ions. This results in re-sputtering effects andsince the field is stronger at the outer diameter, the acceleration willbe correspondingly greater at the outer diameter. Re-sputtering effectsduring film growth have been discussed by D. W. Hoffman, “IntrinsicRe-sputtering—Theory and Experiment”, (J. Vac. Sci. Tech. A(8), 3707,(1990)). The intrinsic re-sputtering efficiency was found to stronglydepend on the mass ratios of the target material (Mt) and the sputteringgas (Mg) and shows mostly a linear dependence on the followingdimensionless parameter: (Mt−Mg)/(Mt+Mg). Experimentally it is foundthat Ar ions (mass=40 amu) re-sputter Pt (195 amu) more effectively thanCo (59 amu) or Cr (52 amu). The re-sputtering efficiency is also foundby Hoffman to depend on the energy of the sputtering ion. This isprovided by the substrate bias and as discussed it is largest at theouter diameter (OD) in the embodiment being described. Therefore, it isexpected that platinum will be re-sputtered more effectively by theionized sputtering Ar+ gas where the electric field is greater. Thisdifferential re-sputtering will lead to a reduced platinum content wherethe electric field is greatest. Since the electric field in the carrierexample above, declines along the radius of the disk 16, the platinumcontent is predicted to increase along radial lines from the outerdiameter (circumference) and the inner diameter.

[0020] Experimental data of the platinum content of a sputtered disk 16plotted by radial distance is given in FIG. 6. The disk 16 was supportedin the method shown for carrier 41 in a multidisk pass throughsputtering system such as those commercially available from the Ulvaccompany. The substrate 26 was AlMg with a NiP coating. The underlayerwas CrV. The magnetic film and the targets were CoPtCrTa; The biasvoltage was about −300 v. The platinum content was measured bymicroprobe (the squares on the graph) and XRF (the circles). Each methodconfirmed the clear trend of increasing platinum content from the outercircumference to the inner diameter. The total increase in the platinumcontent along a 35 mm radial line was approximately 2 at. %, i.e. fromabout 9 to 11 at. %. The slope of the platinum content could be expectedto reverse if the voltage source is shifted to the inner diameter of thedisk

[0021]FIG. 7 is a plot of the coercivity (Hc) versus the platinumcontent for the same disk of FIG. 6. For the 2 at. % increase ofplatinum content the data show an increase of Hc from about 2550 Oe to2800 Oe as confirmation of the relationship between coercivity andplatinum content.

[0022] The method of varying the electric field distribution across thesurface of the disk is not limited to linearly increasing or decreasingfields. Special bias contacts and geometries across the surface of thedisk can be used to produce different bands and/or sectors with varyingcompositional properties. For example, a concentric array of biascontacts each of which is held at a selected potential can be broughtinto contact with the substrate to modulate the electric fielddistribution across the disk surface. In the embodiment described indetail above results in modulation of the electric field along radiallines; however, using an array of bias contacts would allow themodulation to occur circumferentially as well. Variations of such anapproach could allow servo islands or sectors with large modulations ofcoercivity to be distributed circumferentially. Patterned media for usein disk drives have also been proposed as a way of decreasing the trackwidth. Using the method of the invention patterned magnetic media couldbe produced by tailoring the electric field distribution across thesurface of the disk.

[0023]FIG. 8 is an illustration of a nonuniform electric field along themidline section of a substrate 17 applied with a plurality of electrodes67 according to the invention. The previous illustration used a singleelectrode which was also the carrier. These two function need not beprovided by the same member. The electrodes 67 in FIG. 8 contact thesubstrate 17 on one surface and generate the electric field on theopposite surface where the deposition is occurring. The electrodes 67are shown as point contact elements, but can be any shape. Theelectrodes 67 can be connected to independent power sources to alloweach one to have a potential independent of the other electrodes and,therefore, to allow maximum flexibility in tailoring the variations inthe electric field for the particular application. The electrodes 67 ofthe embodiment of FIG. 8 can also be arranged in any x-y pattern in theplane of the substrate.

[0024] In any application of the invention the substrate is not requiredto be conductive except for the surface. Thus, nonmetallic materialssuch as glass may be used with the method of the invention if a layer ofconductive material is applied to the surface. The conductive layer maybe sputtered or deposited by other means. If the bias is applied usingan RF signal, there is no need for the surface to be conductive.

[0025] It is well known in the sputtering art that variations insputtering systems make it difficult to make quantitative predictions;therefore, the experimental data given above should be used forqualitative understanding of the technique.

[0026] The ability to affect the composition of a thin film according tothe mass of the elements by an applied electric field can conceptuallybe applied to nonmagnetic films and to nondisk-shaped substrates. If theelectric field can be varied over distance or time and the masses of theelements are sufficiently different, the technique of the invention maybe employed.

1. A method of sputtering a thin film comprising the steps of:installing a target containing first and second elements in a sputteringchamber, the first element having a higher atomic weight than the secondelement; placing a substrate with a conductive surface in electriccontact with at least a first electrode; generating a plasma containingpositive ions of first and second elements; and differentiallyre-sputtering the first element by applying a first electric potentialto the first electrode to form a nonuniform electric field along theconductive surface to deposit the thin film with variations in an atomicpercentage of the first element along the conductive surface.
 2. Themethod of claim 1 wherein the substrate is a disk having a central holeand the first electrode contacts the conductive surface at an outerdiameter and the nonuniform electric field varies monotonically alongradial lines on the conductive surface of the disk.
 3. The method ofclaim 1 wherein the first element is platinum, the second element iscobalt and the substrate is a disk.
 4. The method of claim 3 wherein thefirst electrode contacts the disk around a circumference of the disk. 5.The method claim 4 wherein the nonuniform electric field variesaccording to radial position on the disk and the variation in the atomicpercentage of platinum is along radial lines on the disk.
 6. The methodof claim 5 wherein the atomic percentage of platinum is lowest at thecircumference of the disk.
 7. The method of claim 2 wherein the thinfilm is magnetic and has a coercivity gradient along radial lines on thedisk.
 8. The method of claim 7 wherein the coercivity is lowest at acircumference of the disk.
 9. The method claim 1 further comprising thesteps of placing the conductive surface in electric contact with asecond electrode; and applying a second electric potential to the secondelectrode, the second electric potential being different from the firstelectric potential.
 10. The method claim 9 further comprising the stepsof placing the conductive surface in electric contact with a thirdelectrode; and applying a third electric potential to the thirdelectrode, the third electric potential being different from the firstand second electric potentials.
 11. The method claim 1 furthercomprising the steps of placing the conductive surface in electriccontact with a plurality of electrodes arranged in a pattern andapplying nonuniform electric potentials to the plurality of electrodesto modulate the electric field distribution across the disk surface toproduce a pattern in the variations in an atomic percentage of the firstelement along the conductive surface.
 12. The method claim 11 whereinthe plurality of electrodes are arranged in a concentric array.
 13. Themethod claim 11 wherein the atomic percentage of the first elementvaries circumferentially.
 14. The method claim 13 wherein the variationsin an atomic percentage of the first element form servo islands whichare distributed circumferentially.
 15. An article of manufacturecomprising: a substrate; and a thin film on a surface of the substrateincluding at least first and second elements with an atomic percentageof the first element varying systematically along lines on the surface.16. The article of claim 15 wherein the substrate is a disk, the firstelement is platinum, the second element is cobalt, the thin film ismagnetic and the atomic percentage of platinum varies according toradial position on the surface.
 17. The article of claim 16 wherein themagnetic thin film has a coercivity which varies according to radialposition on the surface.
 18. The article of claim 16 wherein the atomicpercentage of platinum is lowest at a circumference of the disk.
 19. Thearticle of claim 16 wherein the atomic percentage of platinum is highestat a circumference of the disk.
 20. The article of claim 15 wherein thesubstrate is a disk and the atomic percentage of the first element isequal in concentric bands on the disk forming a pattern.
 21. The articleof claim 15 wherein the substrate is a disk and the atomic percentage ofthe first element varies circumferentially.
 22. The article of claim 15wherein the substrate is a disk, the thin film is magnetic and thevariations in an atomic percentage of the first element form servoislands which are distributed circumferentially.
 23. A disk drivecomprising: a magnetic transducer including a read and a write head; aspindle; and a magnetic thin film disk mounted on the spindle, themagnetic thin film disk including cobalt and platinum, and the magneticthin film having a systematic pattern of variation in an atomicpercentage of platinum.
 24. The disk drive of claim 23 wherein themagnetic thin film has a radial gradient in coercivity corresponding tothe radial gradient in the atomic percentage of platinum.
 25. The diskdrive of claim 23 wherein the atomic percentage of platinum is lowest ata circumference of the disk.
 26. The disk drive of claim 23 wherein theatomic percentage of platinum is highest at a circumference of the disk.27. The disk drive of claim 23 wherein the atomic percentage of platinumis equal in concentric bands on the disk forming a pattern.
 28. The diskdrive of claim 23 wherein the atomic percentage of platinum variescircumferentially.
 29. The disk drive of claim 23 wherein the variationsin an atomic percentage of platinum form servo islands which aredistributed circumferentially.
 30. A method of sputtering a thin filmcomprising the steps of: installing a target containing first and secondelements in a sputtering chamber, the first element having a higheratomic weight than the second element; generating a plasma containingpositive ions of first and second elements; and placing a substrate inelectric contact with at least a first electrode; differentiallyre-sputtering the first element by applying a RF electric potential tothe first electrode to form a nonuniform electric field along a surfaceof the substrate to deposit the thin film with variations in an atomicpercentage of the first element along the surface.
 31. The method ofclaim 30 wherein the substrate is a disk having a central hole and thefirst electrode contacts the conductive surface at an outer diameter.32. The method of claim 30 wherein the first element is platinum, thesecond element is cobalt and the substrate is a disk.
 33. The methodclaim 32 wherein the nonuniform electric field varies according toradial position on the disk and the variation in the atomic percentageof platinum is along radial lines on the disk.
 34. The method of claim32 wherein the thin film is magnetic and has a coercivity gradient alongradial lines on the disk.
 35. The method of claim 32 wherein thesubstrate is a disk, the nonuniform electric field varies along radiallines on the disk, the variation in the atomic percentage of the firstelement is along the radial lines, the thin film is magnetic and has avariation in coercivity corresponding to the variation in the atomicpercentage of the first element.
 36. The method of claim 30 wherein thesubstrate is a disk and the atomic percentage of the first element isequal in concentric bands on the disk forming a pattern.
 37. The methodof claim 30 wherein the substrate is a disk and the atomic percentage ofthe first element varies circumferentially.
 38. The method of claim 30wherein the substrate is a disk, the thin film is magnetic and thevariations in an atomic percentage of the first element form servoislands which are distributed circumferentially.